Method for determining the ignitability of fuel with an unknown fuel quality

ABSTRACT

The invention relates to a method for determining the ignitability of fuel, particularly of diesel, biodiesel, gas-to-liquid or biomass-to-liquid fuel, with an unknown fuel quality for an internal combustion engine. Provision is made for the density of the fuel to be ascertained and for the ignitability to be derived from this.

The invention relates to a method for determining the ignitability offuel, particularly diesel, biodiesel, gas-to-liquid (GTL) orbiomass-to-liquid (BTL) fuel, with an unknown fuel quality, which isspecified for use in an internal combustion engine.

TECHNICAL FIELD

Fuels for diesel engines of motor vehicles, such as diesel, biodiesel orgas-to-liquid fuel, i.e. liquid fuel obtained from natural gas,partially have very different fuel qualities. Particularly theignitability of the fuel, which is very important for the combustion inthe cylinders of diesel engines and is usually expressed as the cetaneindex CCI or the cetane number, can vary considerably for differentfuels, a bandwidth of the cetane index being definitely possible betweenvalues of 38 for diesel fuel in the USA and approximately 70 for GTLfuel.

BACKGROUND

In light of such a large range of fuel qualities, it is no longerpossible with regard to the operation of an internal combustion engineat every engine operating point to find suitable parameters for theopen-loop or closed-loop control of the combustion in the cylinders, asfor example the quantity of injected fuel, the point in time of pilotand main injections, the rail pressure of a common rail fuel injectionsystem, the quantity of fresh air supplied to the cylinders, thesupercharging pressure in supercharged internal combustion engines, thethrottle valve position etc., with which good results with regard toemissions and drivability of the motor vehicle can be obtained for allof the fuel qualities.

That is why practical methods are needed, with which the ignitability ofthe fuels used in the operation of the internal combustion engine,respectively the motor vehicle, can be ascertained with sufficientaccuracy.

Such methods have already been proposed, wherein the quality andparticularly the ignitability of the fuel being used is derived from theprogression of the cylinder pressure. What is considered a disadvantagein these methods is, however, that the installation of an additionalcylinder pressure sensor is required and that the measurement of thecylinder pressure can only be performed in a special operating mode ofthe internal combustion engine.

Based on this fact, the task underlying the invention is to improve amethod of the kind mentioned at the beginning of the application to suchan extent that the determination of the ignitability of fuel with anunknown fuel quality can be performed during the normal operation of theinternal combustion engine and without necessitating the use ofadditional sensors.

SUMMARY

The task is thereby solved according to the invention, in that thedensity of the fuel is ascertained, and the ignitability of the fuel isderived from this.

The idea behind the invention is that the cetane index, which serves asa measurement for the ignitability of fuels for diesel engines, canaccording to ASTN D976 be determined by empirical formulas, into whichonly the distillation range, respectively a corrected average boilingtemperature of the fuel, besides the density ρ of the fuel at aspecified temperature goes. Because the distillation range, respectivelythe corrected average boiling temperature of the fuel, for all of thefuels used in diesel engines can be assumed as approximately constant, adefinite relationship consequently ensues between the density ρ of thefuel and the cetane index CCI, so that this can be calculated when thedensity ρ is known.

While a determination of the fuel density from the mass and the volumeof the fuel contained in a fuel tank of the internal combustion engineis not possible without additional sensors and in addition is largelyinaccurate due to the fluctuating fluid levels, the mass as well as thevolume of the fuel injected into a cylinder of the internal combustionengine can be obtained from information or data, which are required forthe closed-loop or open-loop control of the combustion in the cylindersof modern motor vehicle engines and consequently are as a rule alreadyprovided by indicators, sensors or probes present in the internalcombustion engine, respectively the motor vehicle.

Provision is, for example, made in a preferred configuration of theinvention for the mass of the fuel, which is delivered into a cylinderof the internal combustion engine during an injection, to be determinedfrom the mass of the fresh air delivered into this cylinder forcombustion and the combustion-air ratio (air ratio) λ in the combustionexhaust gas discharged from the cylinder, in that the combustion-airratio λ is measured during a time period, wherein the injected fuelquantity and the quantity of fresh air being supplied are maintained ata constant level.

In internal combustion engines with a common rail fuel injection system,the volume of the fuel delivered into a cylinder of the internalcombustion engine during one injection does not have to be ascertainedwith sensors but can be calculated according to a further preferredconfiguration of the invention from the rail pressure as well as fromthe duration of the activation of the injectors of the fuel injectionsystem. This results from the fact that the volume of the injected fuelis established by these variables. The backpressure in the cylinder andthe temperature of the fuel measured by a temperature sensor of the fuelinjection system are, however, thereby preferably taken into accountbecause the accuracy of the calculation of the injected fuel volume fromthe rail pressure as well as from the duration of the activation of theinjectors can be improved in this way.

Because in diesel internal combustion engines, the mass of the fresh airdelivered into the cylinder, which is measured by an air mass sensor ofthe internal combustion engine, and the combustion-air ratio, which ismeasured by a lambda probe in the exhaust gas tract of the internalcombustion engine, are normally transferred in the form of sensorsignals with data to an engine control unit of the internal combustionengine for evaluation, it is particularly advantageous if at least thedetermination of the mass of the fuel injected into the cylinder of theinternal combustion engine is performed on the basis of these data.Because the determination of the fuel volume delivered into a cylinderof the internal combustion engine for one injection takes placemathematically, it can, however, likewise be advantageously performed bya processor of the engine control unit of the internal combustion enginejust like the ascertainment of the density of the fuel from mass andvolume as well as the mathematical derivation of the ignitability,respectively of a cetane index, from the calculated density of the fuel.

An absolute value for the mass and/or for the density of the fuel ispreferably ascertained on the basis of models in the engine controlunit. A reference value for the mass and/or for the density of theinjected fuel can, however, alternatively or additionally be determinedfor a fuel with a known fuel quality and can be compared inpredetermined time intervals with the mass and/or the density of theinjected fuel, which were ascertained for the same fuel. This is done inorder that allowance is made for deviations due to deterioration whenascertaining the mass of the fuel and/or the fresh air mass deliveredinto the cylinder.

Because rapid changes in the density of the fuel can suggest the use ofa fuel with a different fuel quality, the density of the fuel isadvantageously ascertained before a filling of the fuel tank (fueling).In so doing, a filling of the tank (fueling) can be suggested from achange in the fill level of the fuel in a fuel tank, an opening of afuel filler flap and/or a calculation of the fuel consumption of theinternal combustion engine.

If the occasion arises that the density of the fuel being used andthereby the ignitability of the fuel, which was ascertained via thedensity, have changed, a portion of the significant parametersdesignated for the open-loop or the closed-loop control of thecombustion in the internal combustion engine, such as air mass,initiation of activation and rail pressure, or nominal valuecharacteristic diagrams of these parameters is changed in such a waythan an optimal combustion once again takes place.

BRIEF DESCRIPTION OF THE DRAWING

In the following, the invention is explained in detail using one of theexamples of embodiment depicted in the drawing. The following are shown:

FIG. 1 is a schematic view of parts of a motor vehicle with a dieselengine;

FIG. 2 is a block diagram of a method for determining the ignitabilityof fuel for the diesel engine according to a first variation; and

FIG. 3 is a block diagram of a modification of the method from FIG. 2.

DETAILED DESCRIPTION

The motor vehicle 2 schematically depicted in FIG. 1 of the drawing hasa diesel engine 6, which is selectively supplied with diesel, biodieselor gas-to-liquid (GTL) fuel from a fuel tank 4, with a common rail fuelinjection system 8, which is activated by an engine control unit 10.

The diesel engine 6 has in a known manner an intake tract 12 with an airmass flow meter 14 for measuring the fresh air mass delivered into thecylinders 16 of the engine 6 and a throttle valve 18 for controlling thefresh air supply as well as an exhaust gas tract 20 with a lambda probe22 for measuring the combustion-air ratio (air ratio λ) of thecombustion gases discharged from the cylinders 16.

The fuel injection system 8 comprises likewise in a known manner aplurality of injectors or injection valves 24, which are supplied withfuel from the fuel tank 4 by a fuel pump 26 and a high pressure pistonpump 28 via a common manifold (rail) 30. The fuel line 32 leading fromthe fuel tank 4 to the fuel distributor rail 30 comprises besides a fuelfilter 34 and both pumps 26, 28 additionally a pressure control valve 36for controlling the fuel pressure before the high pressure pump 28, atemperature sensor 38 for measuring the fuel temperature as well as apressure sensor 40 for measuring the pressure in the fuel distributorrail 30 (rail pressure).

The engine control unit 10 is connected via signal lines 42 to thesensors 38,40 and the injectors 24 of the fuel injection system 8, tothe air mass flow meter 14 and an actuator 44 for adjusting the throttlevalve 18, to the lambda probe 22 as well as to further, unspecifiedsensors and actuators, as, for example, a tachometer, respectively anaccelerator pedal of the motor vehicle 2.

Sensor signals with information or data are evaluated in the enginecontrol unit 10. Said data comprise among other things the fresh airmass delivered into the cylinders 16, which is measured by the air massflow meter 14, the combustion-air ratio measured by the lambda probe 22,the rail pressure measured by the pressure sensor 40 as well as the fueltemperature measured by the temperature sensor 38. The evaluatedinformation or data together with an actuating signal from theaccelerator pedal as well as with open-loop or closed-loop variablesdeposited in characteristic diagrams provide for the open-loop,respectively closed-loop, control of diverse parameters, which aresignificant for the combustion of fuel in the cylinders 16. Theseparameters can comprise among other things the quantity of the fueldelivered into the cylinders 16 during one or a plurality of pilotinjections, the point in time of the initiation of the pilot injection,respectively the pilot injections, the temporal interval between thepilot injections, respectively between a pilot injection, and the maininjection, the height of the rail pressure supplied by the high pressurepump 28, the position of the throttle valve 18, the swirl of theinjected fuel, the exhaust gas recirculation rate, i.e. the quantity ofthe exhaust gas recirculated out of the exhaust gas tract 20 into thecylinders, as well as the supercharging pressure of a super charger inthe case of supercharged diesel engines.

A portion of the information, which is transferred from the measuringdevices, probes, respectively sensors 14, 22, 38, 40, to the enginecontrol unit 10, is furthermore used in the normal driving operation ofthe motor vehicle 2 in order to ascertain the ignitability of the fueldelivered from the fuel tank 4 into the fuel injection system 8 withoutnecessitating the use of additional sensors or a special operating mode.Said ignitability can undergo considerable changes particularly when achange in the type of fuel is made, for example from diesel fuel tobiodiesel fuel or vice versa. The ascertained ignitability of the fuel,which represents a measurement for its fuel quality and is usuallyexpressed as the cetane index CCI or cetane number, is then used by theengine control unit 10 in order if required, i.e. when a change in theascertained ignitability occurs, to appropriately change a portion ofthe previously mentioned parameters, respectively their nominal valuecharacteristic diagrams, which are significant for the combustion in thecylinders, so that once again an optimal combustion is achieved.

Because the cetane index can on the one hand be determined with the aidof empirical formulas, for example according to an especially simpleformula:CCI=454.74−1.641416ερ+0.00077474ερ2−0.554εt50+97.803 log 2(t50)whereby the cetane index CCI besides from being a function of theaverage boiling temperature t50 of the fuel in ° C., corrected to theICAO Standard Atmosphere, is only a function of the density ρ of thefuel in kg/m³ at a temperature of 15° C. and because on the other hand,the average boiling temperature t50 for all of the fuels, which arenormally used in diesel engines, can be considered approximately asconstant, it is possible to calculate the cetane index CCI with asufficient degree of accuracy solely from the density ρ of the fuel.

A method suited for this purpose is schematically depicted in FIG. 2,wherein the density ρ of the fuel is calculated as an absolute valuefrom the quotient of the mass m and the volume V of the fuel deliveredduring one injection into one of the cylinders 16 of the engine 6.

In the process, the mass m of the injected fuel is determined using asuitable software in the engine control unit 10 in step S1 with the aidof a model calculation constructed for this purpose from the mass of thefresh air 46 delivered for combustion into the cylinder, which ismeasured by the air mass flow meter 14, and from the combustion-airratio 48 (air ratio λ) in the combustion exhaust gas being dischargedfrom the cylinder 16; while in step S2, the injected fuel volume V,which is established by the rail pressure, as measured by sensor 40, andby the activation duration 52 of the injectors, is likewise determinedwith the aid of a model calculation. In order to improve the accuracy ofthe method, the fuel temperature 54 measured by the sensor 38 as well asthe backpressure 56 in the cylinder 16 deposited for the respectiveoperating point of the engine 6 in characteristic diagrams of the enginecontrol unit 10 are additionally taken into account in step S2.

After the density ρ of the injected fuel has been calculated as thequotient of the mass m and the volume V in step S3, the cetane index ofthe fuel is then calculated in step S4 from this density ρ, for exampleusing the formula stated above.

Because the fuel mass m, which was actually injected, can slowly changeat constant activation parameters, as for example rail pressure 50,activation duration 52, fuel temperature 54 and backpressure 56 in thecylinder, as the result of drifts in the fuel injection system 8 overthe service life of the engine 6, respectively the motor vehicle 2 justas the air mass 46 measured by the air mass flow meter 14 as a result ofa drift of the air mass flow meter 14, it is possible that the absolutevalue of the density ρ and the cetane index determined from saidabsolute value can gradually deviate from the actual density ρ,respectively from the actual cetane index CCI, of the fuel.

In order to compensate for this gradual deviation, respectively in orderto take said deviation into account when ascertaining the density ρ, areference value 58 for the mass m of the injected fuel can be determinedin a preceding step S0 for a new motor vehicle with a fuel with a knowncetane index in the manner previously described. In step 5 while usingthe same fuel, this reference value 58 can then be compared with thefuel mass m, which was ascertained (FIG. 2), in predetermined temporalintervals, for example during each service inspection of the motorvehicle 2. This is done as shown in FIG. 3 in order to derive acorrection value 60 for the calculation of the cetane index CCI. Thiscorrection value is then taken into account in step S4.

On the contrary, rapid changes in the cetane index CCI, which wasascertained, suggest rather a change in the density ρ of the fuel, sothat the accuracy of the method previously described can thereby alsoalternatively or additionally be improved, in that the determination ofthe fuel mass m is correlated with the fuelings of the tank 62. Saidchange in the density ρ of the fuel can, for example, be suggested froma large change in the fill level of the fuel in the fuel tank 4, fromthe opening of a fuel filler flap or from the calculated fuelconsumption.

As a further advantage of the method described, the fuel density ρascertained for the determination of the ignitability can alsofurthermore be taken into account as a multiplicative factor during theconversion of the fuel quantity to be fed into the injectors 24 into theactivation duration 52 of the injectors 24.

1. A method of determining the ignitability of a fuel of an unknownquality for use in an internal combustion engine, wherein the fuel isparticularly a diesel fuel, a biodiesel fuel, a gas-to-liquid fuel, or abiomass-to-liquid fuel, the method comprising: on a control unitconnected to the internal combustion engine, determining a mass of fuelinjected into at least one of a plurality of cylinders of the internalcombustion engine from a mass of fresh air delivered for combustion intothe at least one cylinder and from a combustion-air ratio in acombustion exhaust gas discharged from the at least one cylinder;determining a volume of the fuel injected into the at least onecylinder; calculating a density of the fuel from the determined mass ofthe injected fuel and the determined volume of the fuel injected; andderiving the ignitability of the fuel from the fuel density.
 2. A methodaccording to claim 1, further comprising calculating the volume of fuelinjected into the at least one cylinder from a rail pressure of a commonrail fuel injection apparatus and from a duration of an activation of aplurality of injectors of the fuel injection apparatus.
 3. A methodaccording to claim 2, further comprising accounting for a backpressurein the at least one cylinder and/or a temperature of the injected fuelwhen calculating the volume of fuel injected into the at least onecylinder.
 4. A method according to claim 3, further comprising measuringthe temperature of the injected fuel with a temperature sensor of thefuel injection apparatus.
 5. A method according to claim 1, furthercomprising ascertaining an absolute value for the mass and/or for thedensity of the fuel on a basis of one or more models.
 6. A methodaccording to claim 1, further comprising: determining a reference valuefor the mass and/or for the density of the injected fuel from a fuelwith a known fuel quality; and comparing the reference value with theascertained mass and/or the density of the injected fuel, wherein thecomparison is made at a predetermined temporal interval.
 7. A methodaccording to claim 1, further comprising ascertaining the density of thefuel either prior to or after a filling of a fuel tank or both.
 8. Amethod according to claim 7, further comprising deriving an occurrenceof the filling of the fuel tank from a change in a fill level of thefuel in the fuel tank, an opening of a filler flap and/or a calculatedfuel consumption of the internal combustion engine.
 9. A methodaccording to claim 1, further comprising changing as a function of theascertained fuel ignitability one or more parameters for the open-loopand/or closed-loop control of fuel combustion in a plurality ofcylinders of the internal combustion engine or a nominal valuecharacteristic diagram of the one or more parameters.
 10. A methodaccording to claim 9, wherein the one or more parameters comprise of anair mass, an initiation of activation and a rail pressure.